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Applying the decorator pattern in .NET Core using Castle.DynamicProxy

In this post I want to explain what the decorator pattern is, why it is useful and how you can implement it in a generic way using Castle.DynamicProxy.

What is the Decorator pattern?

Before we dive into the technical details, let us start with a recap about what the decorator pattern actually is:

Decorator is a structural design pattern that lets you attach new behaviors to objects by placing these objects inside special wrapper objects that contain the behaviors.

There are a lot of use cases where the decorator pattern can help, in this blog post I focus on one of them; using a decorator to handle cross cutting concerns(caching, error handling, logging, …) without polluting your original object.

I’ll show you an example where I introduce caching at the repository interface level.

How to integrate the decorator pattern in a generic way?

You don’t need a special library or anything to implement the decorator pattern in C#. However I would like to write it in a generic way without the need to create the same decorator over and over again for different interfaces. Therefore I will use Castle.DynamicProxy. It allows me to create lightweight .NET proxies on the fly at runtime. These proxy objects allow calls to members of an object to be intercepted without modifying the code of the class. These interception points are the perfect place to introduce our caching behavior.

We’ll start by creating our cache interceptor using Castle.DynamicProxy:

This interceptor only calls our async cache interceptor that contains the real caching logic:

In this async interceptor we use an IMemoryCache instance to cache the data and use the intercepted method name together with the method arguments to construct the cache key. This guarantees that when our method is called with different arguments the correct values are returned.

We register these interceptors in the DI container and also register the ProxyGenerator singleton that is responsible for the proxy generation:

The last step is to write the logic that wraps the object we want to create in the proxy and plugin our interceptor. As I want to do this in a generic way I have created extension methods on IServiceCollection:

There is only one thing left to do, we call one of this extension methods to register the object that should be proxied:

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